Lithium secondary batteries have features such as high energy density and long service life. Therefore, lithium secondary batteries are widely used as power supplies for electric appliances such as video cameras; portable electronic devices such as laptop computers and mobile telephones, and electric tools such as power tools. Recently, lithium secondary batteries are also applied to large-sized batteries that are mounted in electric vehicles (EV), hybrid electric vehicles (HEV) and the like.
A lithium secondary battery is a secondary battery having a structure in which, at the time of charging, lithium begins to dissolve as ions from a positive electrode and moves to a negative electrode to be stored therein, and at the time of discharging, lithium ions return from the negative electrode to the positive electrode, and it is known that the higher energy density of the lithium secondary battery is attributable to the electric potential of the positive electrode material.
Known examples of this kind of positive electrode active material for lithium secondary batteries include lithium transition metal oxides having a layered structure, such as LiCoO2, LiNiO2, and LiMnO2; and lithium transition metal oxides having a manganese-based spinel structure (Fd-3m), such as LiMn2O4 and LiNi0.5Mn1.5O4 (in the present invention, referred to as “manganese spinel-type lithium transition metal oxides”).
Manganese spinel-type lithium transition metal oxides are inexpensive in terms of raw material cost, are non-toxic and highly safe, and have a nature resistant to overcharge. Therefore, attention has been paid to the lithium transition metal oxides as the next-generation positive electrode active materials for large-sized batteries for electric vehicles (EV), hybrid electric vehicles (HEV) and the like. Furthermore, spinel type lithium transition metal oxides (LMO) capable of three-dimensional insertion and extraction of Li ions have superior power output characteristics compared with lithium transition metal oxides having a layered structure, such as LiCoO2. Therefore, it is expected to use the spinel type lithium transition metal oxides in applications where excellent power output characteristics are required, as in batteries for EV, batteries for HEV, and the like.
Among them, it is know that when a portion of Mn sites in LiMn2O4 is substituted with other transition metals (Cr, Co, Ni, Fe or Cu), the lithium transition metal oxide acquires an operating potential at near 5 V. Thus, currently, development of a manganese spinel-type lithium transition metal oxide having an operating potential of 4.5 V or more (5 V class) is in active progress.
For example, Patent Document 1 discloses, as a positive electrode active material for lithium secondary batteries exhibiting an electromotive force of 4.5 V or more (5 V class), a high capacity spinel type lithium manganese composite oxide positive electrode active material obtained by adding chromium as an essential additive component to a spinel type lithium manganese composite oxide, and further adding nickel or cobalt thereto.
Patent Document 2 discloses a crystal having a spinel structure, LiMn2-y-zNiyMzO4 (provided that M: at least one selected from the group consisting of Fe, Co, Ti, V, Mg, Zn, Ga, Nb, Mo and Cu; 0.25≦y≦0.6, and 0≦z≦0.1), with which charging and discharging is conducted against Li metal at a potential of 4.5 V or more.
Patent Document 3 discloses a spinel type lithium manganese composite oxide represented by Lia(MxMn2-x-yAy)O4 (wherein 0.4<x, 0<y, x+y<2, 0<a<1.2; M includes one or more metal elements selected from the group consisting of Ni, Co, Fe, Cr and Cu, and include at least Ni; and A includes at least one metal element selected from Si and Ti, provided that when A includes only Ti, the value of the proportion of A, y, is such that 0.1<y), as a positive electrode material for higher energy density lithium ion secondary batteries having a high voltage of 4.5 V or more against Li.